Space time block coded transmit antenna diversity for WCDMA

ABSTRACT

A mobile communication system is designed with an input circuit coupled to receive a first plurality of signals (r j (i+τ j ), i=0−N−1) during a first time (T 0 -T 1 ) from an external source and coupled to receive a second plurality of signals (r j  (i+τ j ), i=N−2N−1) during a second time (T 1 -T 2 ) from the external source. The input circuit receives each of the first and second plurality of signals along respective first and second paths (j). The input circuit produces a first input signal (R j   1 ) and a second input signal (R j   2 ) from the respective first and second plurality of signals. A correction circuit is coupled to receive a first estimate signal (α j   1 ), a second estimate signal (α j   2 ) and the first and second input signals. The correction circuit produces a first symbol estimate ({tilde over (S)} 1 ) in response to the first and second estimate signals and the first and second input signals. The correction circuit produces a second symbol estimate ({tilde over (S)} 2 ) in response to the first and second estimate signals and the first and second input signals.

This application is a divisional of prior application Ser. No.10/601,866, filed Jun. 23, 2003, currently pending;

Which was a continuation of prior application Ser. No. 09/205,029, filedDec. 3, 1998; now U.S. Pat. No. 6,643,338, granted Nov. 4, 2003;

Which claimed priority from Provisional Application No. 60/103,443,filed Oct. 7, 1998.

FIELD OF THE INVENTION

This invention relates to wideband code division multiple access (WCDMA)for a communication system and more particularly to space time blockcoded transmit antenna diversity for WCDMA.

BACKGROUND OF THE INVENTION

Present code division multiple access (CDMA) systems are characterizedby simultaneous transmission of different data signals over a commonchannel by assigning each signal a unique code. This unique code ismatched with a code of a selected receiver to determine the properrecipient of a data signal. These different data signals arrive at thereceiver via multiple paths due to ground clutter and unpredictablesignal reflection. Additive effects of these multiple data signals atthe receiver may result in significant fading or variation in receivedsignal strength. In general, this fading due to multiple data paths maybe diminished by spreading the transmitted energy over a wide bandwidth.This wide bandwidth results in greatly reduced fading compared to narrowband transmission modes such as frequency division multiple access(FDMA) or time division multiple access (TDMA).

New standards are continually emerging for next generation wideband codedivision multiple access (WCDMA) communication systems as described inProvisional U.S. Patent Application No. 60/082,671, filed Apr. 22, 1998,and incorporated herein by reference. These WCDMA systems are coherentcommunications systems with pilot symbol assisted channel estimationschemes. These pilot symbols are transmitted as quadrature phase shiftkeyed (QPSK) known data in predetermined time frames to any receiverswithin range. The frames may propagate in a discontinuous transmission(DTX) mode. For voice traffic, transmission of user data occurs when theuser speaks, but no data symbol transmission occurs when the user issilent. Similarly for packet data, the user data may be transmitted onlywhen packets are ready to be sent. The frames include pilot symbols aswell as other control symbols such as transmit power control (TPC)symbols and rate information (RI) symbols. These control symbols includemultiple bits otherwise known as chips to distinguish them from databits. The chip transmission time (T_(C)), therefore, is equal to thesymbol time rate (T) divided by the number of chips in the symbol (N).

Previous studies have shown that multiple transmit antennas may improvereception by increasing transmit diversity for narrow band communicationsystems. In their paper New Detection Schemes for Transmit Diversitywith no Channel Estimation, Tarokh et al. describe such a transmitdiversity scheme for a TDMA system. The same concept is described in ASimple Transmitter Diversity Technique for Wireless Communications byAlamouti. Tarokh et al. and Alamouti, however, fail to teach such atransmit diversity scheme for a WCDMA communication system.

Other studies have investigated open loop transmit diversity schemessuch as orthogonal transmit diversity (OTD) and time switched timediversity (TSTD) for WCDMA systems. Both OTD and TSTD systems havesimilar performance. Both use multiple transmit antennas to provide somediversity against fading, particularly at low Doppler rates and whenthere are insufficient paths for the rake receiver. Both OTD and TSTDsystems, however, fail to exploit the extra path diversity that ispossible for open loop systems. For example, the OTD encoder circuit ofFIG. 5 receives symbols S₁ and S₂ on lead 500 and produces outputsignals on leads 504 and 506 for transmission by first and secondantennas, respectively. These transmitted signals are received by adespreader input circuit (FIG. 6). The input circuit receives the i^(th)of N chip signals per symbol together with noise along the j^(th) of Lmultiple signal paths at a time τ_(j) after transmission. Both here andin the following text, noise terms are omitted for simplicity. Thisreceived signal r_(j) (i+τ_(j)) at lead 600 is multiplied by a channelorthogonal code signal C_(m) (i+τ_(j)) that is unique to the receiver atlead 604. Each chip signal is summed over a respective symbol time bycircuit 608 and produced as first and second output signals R_(j) ¹ andR_(j) ² on leads 612 and 614 as in equations [1-2], respectively. Delaycircuit 610 provides a one-symbol delay T so that the output signals areproduced simultaneously.

$\begin{matrix}{R_{j}^{1} = {{\sum\limits_{i = 0}^{N - 1}\;{r_{j}\left( {i + \tau_{j}} \right)}} = {{\alpha_{j}^{1}S_{1}} + {\alpha_{j}^{2}S_{2}}}}} & \lbrack 1\rbrack \\{R_{j}^{2} = {{\sum\limits_{i = N}^{{2\; N} - 1}\;{r_{j}\left( {i + \tau_{j}} \right)}} = {{\alpha_{j}^{1}S_{1}} - {\alpha_{j}^{2}S_{2}}}}} & \lbrack 2\rbrack\end{matrix}$

The OTD phase correction circuit of FIG. 7 receives the signals R_(j) ¹and R_(j) ² as input signals corresponding to the j^(th) of L multiplesignal paths. The phase correction circuit produces soft outputs orsignal estimates {tilde over (S)}₁ and {tilde over (S)}₂ for symbols S₁and S₂ at leads 716 and 718 as shown in equations [3-4], respectively.

$\begin{matrix}{{\overset{\sim}{S}}_{1} = {{\sum\limits_{j = 1}^{L}\;{\left( {R_{j}^{1} + R_{j}^{2}} \right)\alpha_{j}^{1^{*}}}} = {\sum\limits_{j = 1}^{L}\;{2{\alpha_{j}^{1}}^{2}S_{1}}}}} & \lbrack 3\rbrack \\{{\overset{\sim}{S}}_{2} = {{\sum\limits_{j = 1}^{L}\;{\left( {R_{j}^{1} - R_{j}^{2}} \right)\alpha_{j}^{2^{*}}}} = {\sum\limits_{j = 1}^{L}\;{2{\alpha_{j}^{2}}^{2}S_{2}}}}} & \lbrack 4\rbrack\end{matrix}$

Equations [3-4] show that the OTD method provides a single channelestimate α for each path j. A similar analysis for the TSTD systemyields the same result. The OTD and TSTD methods, therefore, are limitedto a path diversity of L. This path diversity limitation fails toexploit the extra path diversity that is possible for open loop systemsas will be explained in detail.

SUMMARY OF THE INVENTION

These problems are resolved by a mobile communication system comprisingan input circuit coupled to receive a first plurality of signals duringa first time from an external source and coupled to receive a secondplurality of signals during a second time from the external source. Theinput circuit receives each of the first and second plurality of signalsalong respective first and second paths. The input circuit produces afirst input signal and a second input signal from the respective firstand second plurality of signals. A correction circuit is coupled toreceive a first estimate signal, a second estimate signal and the firstand second input signals. The correction circuit produces a first symbolestimate in response to the first and second estimate signals and thefirst and second input signals. The correction circuit produces a secondsymbol estimate in response to the first and second estimate signals andthe first and second input signals.

The present invention improves reception by providing at least 2Ldiversity over time and space. No additional transmit power or bandwidthis required. Power is balanced across multiple antennas.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the invention may be gained by readingthe subsequent detailed description with reference to the drawingswherein:

FIG. 1 is a simplified block diagram of a typical transmitter usingSpace Time Transit Diversity (STTD) of the present invention;

FIG. 2 is a block diagram showing signal flow in an STTD encoder of thepresent invention that may be used with the transmitter of FIG. 1;

FIG. 3 is a schematic diagram of a phase correction circuit of thepresent invention that may be used with a receiver;

FIG. 4A is a simulation showing STTD performance compared to TimeSwitched Time Diversity (TSTD) for a vehicular rate of 3 kmph;

FIG. 4B is a simulation showing STTD performance compared to TSTD for avehicular rate of 120 kmph;

FIG. 5 is a block diagram showing signal flow in an OTD encoder of theprior art;

FIG. 6 is a block diagram of a despreader input circuit of the priorart; and

FIG. 7 is a schematic diagram of a phase correction circuit of the priorart.

FIG. 8 is a space time block coded receiver of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, there is a simplified block diagram of a typicaltransmitter using Space Time Transit Diversity (STTD) of the presentinvention. The transmitter circuit receives pilot symbols, TPC symbols,RI symbols and data symbols on leads 100, 102, 104 and 106,respectively. Each of the symbols is encoded by a respective STTDencoder as will be explained in detail. Each STTD encoder produces twooutput signals that are applied to multiplex circuit 120. The multiplexcircuit 120 produces each encoded symbol in a respective symbol time ofa frame. Thus, a serial sequence of symbols in each frame issimultaneously applied to each respective multiplier circuit 124 and126. A channel orthogonal code C_(m) is multiplied by each symbol toprovide a unique signal for a designated receiver. The STTD encodedframes are then applied to antennas 128 and 130 for transmission.

Turning now to FIG. 2, there is a block diagram showing signal flow inan STTD encoder of the present invention that may be used with thetransmitter of FIG. 1. The STTD encoder receives symbol S₁ at symboltime T and symbol S₂ at symbol time 2T on lead 200. The STTD encoderproduces symbol S₁ on lead 204 and symbol −S₂* on lead 206 at symboltime T, where the asterisk indicates a complex conjugate operation.Furthermore, the symbol time indicates a relative position within atransmit frame and not an absolute time. The STTD encoder then producessymbol S₂ on lead 204 and symbol S₁* on lead 206 at symbol time 2T. Thebit or chip signals of these symbols are transmitted serially alongrespective paths 208 and 210. Rayleigh fading parameters are determinedfrom channel estimates of pilot symbols transmitted from respectiveantennas at leads 204 and 208. For simplicity of analysis, a Rayleighfading parameter α_(j) ¹ is assumed for a signal transmitted from thefirst antenna 204 along the j^(th) path. Likewise, a Rayleigh fadingparameter α_(j) ² is assumed for a signal transmitted from the secondantenna 206 along the j^(th) path. Each i^(th) chip or bit signalr_(j)(i+τ_(j)) of a respective symbol is subsequently received at aremote mobile antenna 212 after a transmit time τ_(j) corresponding tothe j^(th) path. The signals propagate to a despreader input circuit(FIG. 6) where they are summed over each respective symbol time toproduce output signals R_(j) ¹ and R_(j) ² corresponding to the j^(th)of L multiple signal paths as previously described.

Referring now to FIG. 8, there is a space time block coded receiver ofthe present invention. The receiver including despreader circuit 800coupled to receive respective path-specific signals r_(j) (i+τ_(j)) forthe i^(th) chip corresponding to paths j. These path-specific signalsinclude a first input signal from a first antenna ANT 1 (FIG. 2) and asecond input signal from a second antenna ANT2. The first antenna ANT 1(FIG. 2) and a second input signal from a second antenna ANT2. The firstinput signal is transmitted along plural signal paths, each of theplural signal paths having a respective channel characteristic α₁ ¹through α_(j) ¹. The second input signal is also transmitted alongrespective plural signal paths, each having a respective channelcharacteristic α₁ ² through α₁ ². The despreader circuit (FIG. 8)produces and applies respective signals, for example signals R_(j) ¹ andR_(j) ² at leads 82 and 834, to phase correction circuit 810. SignalR_(j) ¹ includes j symbols received at a first time from antenna ANT 1according to equation [5]. Signal R_(j) ² includes j symbols received ata second time from antenna ANT 2 according to equation [6]. The phasecorrection circuit is coupled to receive respective input signals andpath-specific estimate signals, for example inputs signals R_(j) ¹ andR_(j) ², a first plurality of estimate signals and estimate signalsα_(j) ¹* and α_(j) ² at phase correction circuit 810. the phasecorrection circuit produces and applies respective symbol estimatesaccording to equations [7-8], for example first and second symbolestimates S_(j) ¹ and S_(j) ² at leads 836 and 838, to rake combinercircuits 820 and 822. The plurality of first symbol estimates S_(j) ¹correspond to the j signal paths from antenna ANT 1 and include a firstsymbol estimate S₁ ¹. The plurality of second symbol estimates S_(j) ²correspond to the j signal paths from antenna ANT 2 and include a secondsymbol estimate S₁ ². Rake combiner circuit 820 sums first symbolestimates from each path of the phase correction circuit and produces afirst symbol signal {tilde over (S)}₁ at lead 824 according to equation[9]. Likewise, rake combiner circuit 822 sums second symbol estimatesfrom each path of the phase correction circuit and produces a secondsymbol signs {tilde over (S)}₂ at lead 826 according to equation [10].

Referring now to FIG. 3, there is a schematic diagram of a phasecorrection circuit of the present invention that may be used with aremote mobile receiver as in FIG. 8. This phase correction circuitreceives signals R_(j) ¹ and R_(j) ² as input signals on leads 610 and614 as shown in equations [5-6], respectively.

$\begin{matrix}{R_{j}^{1} = {{\sum\limits_{i = 0}^{N - 1}\;{r_{j}\left( {i + \tau_{j}} \right)}} = {{\alpha_{j}^{1}S_{1}} - {\alpha_{j}^{2}S_{2}^{*}}}}} & \lbrack 5\rbrack \\{R_{j}^{2} = {{\sum\limits_{i = N}^{{2\; N} - 1}\;{r_{j}\left( {i + \tau_{j}} \right)}} = {{\alpha_{j}^{1}S_{2}} + {\alpha_{j}^{2}S_{1}^{*}}}}} & \lbrack 6\rbrack\end{matrix}$

The phase correction circuit receives a complex conjugate of a channelestimate of a Rayleigh fading parameter α_(j) ¹* corresponding to thefirst antenna on lead 302 and a channel estimate of another Rayleighfading parameter α_(j) ² corresponding to the second antenna on lead306. Complex conjugates of the input signals are produced by circuits308 and 330 at leads 310 and 322, respectively. These input signals andtheir complex conjugates are multiplied by Rayleigh fading parameterestimate signals and summed as indicated to produce path-specific firstand second symbol estimates at respective output leads 318 and 322 as inequations [7-8].R _(j) ¹α_(j) ¹ *+R _(j) ²*α_(j) ²=(|α_(j) ¹|²+|α_(j) ²|²)S ₁  [7]−R _(j) ¹*α_(j) ² +R _(j) ²α_(j) ¹*=(|α_(j) ¹|²+|α_(j) ²|²)S ₂  [8]

These path-specific symbol estimates are then applied to a rake combinercircuit to sum individual path-specific symbol estimates, therebyproviding net soft symbols as in equations [9-10].

$\begin{matrix}{{\overset{\sim}{S}}_{1} = {{\sum\limits_{j = 1}^{L}\;{R_{j}^{1}\alpha_{j}^{1^{*}}}} + {R_{j}^{2^{*}}\alpha_{j}^{2}}}} & \lbrack 9\rbrack \\{{\overset{\sim}{S}}_{2} = {{\sum\limits_{j = 1}^{L}\;{R_{j}^{1^{*}}\alpha_{j}^{2}}} + {R_{j}^{2}\alpha_{j}^{1^{*}}}}} & \lbrack 10\rbrack\end{matrix}$

These soft symbols or estimates provide a path diversity L and atransmit diversity 2. Thus, the total diversity of the STTD system is2L. This increased diversity is highly advantageous in providing areduced bit error rate. The simulation result of FIG. 4 compares a biterror rate (BER) of STTD with TSTD for various ratios of energy per bit(Eb) to noise (No) at a relative speed of 3 Kmph. The OTD and TSTDsystems were found to be the same in other simulations. The simulationshows that a 7.5 dB ratio Eb/No corresponds to a BER of 2.0E-3 for TSTD.The same BER, however, is achieved with a 7.2 dB ratio Eb/No. Thus, STTDproduces approximately 0.3 dB improvement over TSTD. The simulation ofFIG. 5 compares the BER of STTD with TSTD for various values of Eb/No ata relative speed of 120 Kmph. This simulation shows a typical 0.25 dBimprovement for STTD over TSTD even for high Doppler rates. By way ofcomparison, STTD demonstrates a 1.0 dB advantage over the simulatedcurve of FIG. 5 without diversity at a BER of 2.6E-3. This substantialadvantage further demonstrates the effectiveness of the presentinvention.

Although the invention has been described in detail with reference toits preferred embodiment, it is to be understood that this descriptionis by way of example only and is not to be construed in a limitingsense. For example, several variations in the order of symboltransmission would provide the same 2L diversity. Moreover, theexemplary diversity of the present invention may be increased with agreater number of transmit or receive antennas. Furthermore, novelconcepts of the present invention are not limited to exemplarycircuitry, but may also be realized by digital signal processing as willbe appreciated by those of ordinary skill in the art with access to theinstant specification.

It is to be further understood that numerous changes in the details ofthe embodiments of the invention will be apparent to persons of ordinaryskill in the art having reference to this description. It iscontemplated that such changes and additional embodiments are within thespirit and true scope of the invention as claimed below.

1. A phase correction circuit comprising: A. a first input lead carryinga first signal R_(j) ¹; B. a second input lead carrying a second signalR_(j) ²; C. first multiplier circuitry having one input connected to thefirst lead, a second input receiving a complex conjugate of a firstRayleigh fading parameter estimate signal a_(j) ¹*, and an output; D.second multiplier circuitry having one input connected to the secondlead, a second input receiving the complex conjugate of the firstRayleigh fading parameter estimate signal a_(j) ¹*, and an output; E.first complex conjugate circuitry having an input connected to the firstlead and an output; F. second complex conjugate circuitry having aninput connected to the second lead and an output; G. third multipliercircuitry having one input connected to the output of the first complexconjugate circuitry, a second input receiving a second Rayleigh fadingparameter estimate signal a_(j) ², and an output; H. fourth multipliercircuitry having one input connected to the output of the second complexconjugate circuitry, a second input receiving the second Rayleigh fadingparameter estimate signal a_(j) ², and an output; I. first summingcircuitry having a first positive input connected to the output of thefirst multiplier circuitry, a second positive input connected to theoutput of the fourth multiplier circuitry, and an output providing afirst symbol estimate signal; and J. second summing circuitry having afirst negative input connected to the output of the third multipliercircuitry, a second positive input connected to the output of the secondmultiplier circuitry, and an output providing a second symbol estimatesignal.
 2. The phase correction circuit of claim 1 in which the outputof the first summing circuitry is connected to first rake combinercircuitry and the output of the second summing circuitry is connected tosecond rake combiner circuitry.
 3. The phase correction circuit of claim1 in which the first signal Rj1 equals a_(j) ¹S₁ minus a_(j) ²S₂* andthe second signal R_(j) ² equals a_(j) ¹S₂ plus a_(j) ²S₁*.